Is it wrong to say: "I had learnt that long back"? Is it more apt to say "I had learnt that long ago"?
Using "long ago" is the correct (i.e. commonly heard) term. As seen in this Ngram, "long back" is extremely rare.
That's not to say that there aren't some constructions where "long back" might be correct, but it's quite uncommon. As noted in the comments, the sentence "I learned that a long time back/ago" would be acceptable.